The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to compositions and methods for treating subterranean formations, in particular, oilfield stimulation compositions and methods using polymer crosslinked with metal complexes to viscosify the treatment fluid.
High volumes of formation fracturing and other well treatment fluids are commonly thickened with polymers such as guar gum, the viscosity of which is greatly enhanced by crosslinking with a metal such as chromium aluminum, hafnium, antimony, etc., more commonly a Group 4 metal such as zirconium or titanium. In reference to Periodic Table “Groups,” the new IUPAC numbering scheme for the Periodic Table Groups is used as found in HAWLEY'S CONDENSED CHEMICAL DICTIONARY, p. 888 (11th ed. 1987).
It is well known that metal-crosslinked polymer fluids can be shear-sensitive after they are crosslinked. In particular, exposure to high shear typically occurs within the tubulars during pumping from the surface to reservoir depth, and can cause an undesired loss of fluid viscosity and resulting problems such as screenout. As used herein, the term “high shear” refers to a shear rate of 500/second or more. The high-shear viscosity loss in metal-crosslinked polymer fluids that can occur during transit down the wellbore to the formation is generally irreversible and cannot be recovered. We use the term “persistent gels” herein to refer to such irreversibly crosslinked aqueous polymers.
High shear sensitivity of the metal crosslinked fluids can sometimes be addressed by delaying the crosslinking of the fluid so that it is retarded during the high-shear conditions and onset does not occur until the fluid has exited the tubulars. Because the treatment fluid is initially cooler than the formation and is usually heated to the formation temperature only after exiting the tubulars, some delaying agents work by increasing the temperature at which gelation takes place. Bicarbonate and lactate are examples of delaying agents that are known to increase the gelling temperatures of the metal crosslinked polymer fluids. Although these common delaying agents make fluids less sensitive to high shear treatments, they may at the same time result in a decrease in the ultimate fluid viscosity. Also, the common delaying agents may not adequately increase the gelation temperature for the desired delay, especially where the surface fluid mixing temperature is relatively high or the fluid is heated too rapidly during injection.
The affinity of a ligand to a metal can depend on various conditions, e.g., temperature, pressure, pH, concentrations of the metal and ligand, etc. The data on many metal/ligand binding constants can be found in open literature, for example, R. M. Smith and A. E. Martell, “Critical Stability Constants,” Plenum Press, New York (1989).
Ligands such as triethanolamine, bicine (N,N-bis(2-hydroxyethyl)glycine), lactate, etc., have been used to form metallic complexes yielding aqueous gels of moderate viscosity at the downhole pH and temperature conditions encountered. Bicine and other N-hydroxyalkyl amino acid ligands are disclosed for example in U.S. Pat. No. 4,885,103, U.S. Pat. No. 7,122,690 and US 2007-187,642. Ligands such as bicine have been proposed for use with zirconium as delayed crosslinking agents. However, these ligands can be too strong binding and can result in crosslinking temperatures that are too high in some applications. If the crosslinking is delayed too much, the treatment fluid may not be sufficiently viscosified to initiate and propagate a fracture, or to carry proppant, for example, which can result in screenout and treatment failure.
Another issue with some crosslink delay ligands such as bicine is that they may be sensitive to small pH changes. Small changes in pH, which can occur for a number of reasons during well treatment, can result in dramatic lowering of the crosslinking temperature and premature crosslinking leading to excessive pumping pressures and shear-induced viscosity losses, or in a dramatic increase in the crosslinking temperature and excessive delay of crosslinking leading to screenout or treatment failure.
In some prior art treatment systems, borate crosslinkers have been used in conjunction with metal crosslinkers, e.g. U.S. Pat. No. 4,780,223. In theory, the borate crosslinker can gel the polymer fluid at a low temperature through a reversible crosslinking mechanism that can be broken by exposure to high shear, but can repair or heal after the high shear condition is removed. The shear-healing borate crosslinker can then be used to thicken the fluid during high shear such as injection through the wellbore while the irreversible metal crosslinking is delayed until the high shear condition is passed. A high pH, e.g. 9 to 12 or more, is usually used to effect borate crosslinking, and in some instances as a means to control the borate crosslinking. For example, the pH and/or the borate concentration may be adjusted on the fly in response to pressure friction readings during the injection so that the borate crosslinking occurs near the exit from the tubulars in the wellbore. The metal crosslinker must of course be suitable for use at these pH conditions and must not excessively interfere with the borate crosslinking.
A new way to delay the onset of crosslinking of metal crosslinked polymer fluids, without risk of compromising the fluid performance, is needed. Desirably, the onset of crosslinking would be insensitive or only moderately sensitive to pH changes and compatible with borate crosslinkers.
Zirconium-amino acid complexes are known in antiperspirant actives in the form of gels without a crosslinked polymer and/or in anhydrous systems where they are not used to viscosify aqueous fluids. For example, U.S. Pat. No. 4,148,812 discloses a method of preparing basic zirconium-amino acid complexes for antiperspirant formulations, consisting essentially of reacting a water soluble amino acid salt such as sodium glycinate with a water soluble zirconium salt to form a precipitate comprising a gel. The zirconium complex gel is in turn reacted with aluminum compounds to form the antiperspirant active. U.S. Pat. No. 5,895,644 and WO 1999026598 disclose an anhydrous gel used in an antiperspirant stick composition, wherein the anhydrous gel comprises dibenzylidene sorbitol (DBS), derivatized guar such as hydroxypropyl guar, and a solvent such as alcohol or glycol that does not react in the presence of DBS with an antiperspirant active salt such as aluminum/zirconium salts complexed with a neutral amino acid such as glycine. None of these references disclose a hydrated polymer crosslinked in an aqueous medium with a zirconium-amino acid complex as a delayed viscosity agent.